1. Field of the Invention
The invention relates to actively redundant drive systems for roller mills, which comprise a housing, a rotating grinding table with grinding track, grinding rollers that roll on the grinding track, an axial bearing, and a drive having an electric motor and a gear reducer for driving the grinding track.
2. Description of the Background Art
Roller mills have been known for over 100 years, and are used throughout the world. They exist in an extremely wide variety of designs. Thus, for example, DE 153 958 C from 1902 shows a cone mill with a revolving grinding table on which rest eight grinding cones under spring pressure.
Modern mills use grinding rollers that have heavy weights and large diameters to achieve high milling output. Please see DE 198 26 324 C, DE 196 03 655, which corresponds to U.S. Pat. No. 6,021,968, and also EP 0 406 644 B. This type of roller mill has gained extremely wide acceptance in practice because it has considerable advantages with regard to design, control, and energy economy. The chief areas of application for modern roller mills are the cement industry and coal-fired power plants. In the cement industry, roller mills are used for producing raw cement meal as well as for clinker grinding and coal grinding. In combination with rotary kilns and calcining installations, the furnace exhaust gases from the heat exchanger and clinker cooler can be used to dry the grinding stock and pneumatically transport the ground stock. In power plants, the roller mills are used to finely grind the coal and feed it directly into the boiler with the aid of the classifier air, if possible without the use of an intermediate bunker.
It is a matter of course that even modern roller mills are subject to wear resulting from use. The mill must therefore be shut down for maintenance and repair. In recent years, maintenance and repair in the grinding chamber and on the grinding tools have been optimized to such a degree that these tasks can be carried out within a tolerable period of time.
Modern large mills require drive power levels of up to 10 MW. It is a matter of course that the associated bearings and drives, in particular the transmissions, must be of special design. The teeth, the shaft bearings, the integrated axial thrust bearings and their supports within the transmission housing, are particularly heavily loaded. For drive power levels up to 6 MW, planetary bevel gear transmissions, which are matched to the circular grinding table on account of their circular shape, have become established as the prior art; they transmit the static and dynamic grinding forces to the foundation. For example, see DE 35 07 913 A or DE 37 12 562 C, which corresponds to U.S. Pat. No. 4,887,489. Pivoted-pad bearings with hydrodynamic and/or hydrostatic lubrication are used as axial thrust bearings; please see DE 33 20 037 C.
These designs, space-saving in and of themselves, have significant disadvantages, however. As soon as a problem arises with just one component, the entire drive must be dismantled. It has proven to be particularly disadvantageous in this regard that it is not possible to visually inspect the gears of the planetary transmission. This is not possible until the drive has been completely dismantled. Since these drives are special designs, procurement of replacement parts takes a correspondingly long time, i.e., weeks or months, since stocking of replacement parts is considered too cost-intensive on account of the special designs. This is unsatisfactory.
Another disadvantage of the prior art drive design is what is called the maintenance drive, which rotates the grinding table during certain maintenance and repair operations, but which only functions as long as the primary transmission itself functions.
Naturally, there has been no shortage of proposals for doing away with these inadequacies and disadvantages. Thus, DE 39 31 116 C shows a drive device for a roller mill having a grinding table that can rotate about a vertical axis, which has a crown gear connected to the lower part of the grinding table. Moreover, two diagonally arranged drives are provided, each consisting of a drive motor and a gear reducer. Each gear reducer has two pinions that mesh with the crown gear of the grinding table.
Known from DE 76 20 223 U is a roller mill with a ring gear located under its grinding table. The pinions of four hydraulic motors fastened to the base of the mill housing mesh with the ring gear.
Despite the theoretical advantages of these multiple-motor drive concepts, they have been unable to gain acceptance in practice. The reasons for this are the lower efficiency as compared with drives having electric motors and transmissions, and lower availability and service life of the hydraulic components. The dual-drive concept described was unable to gain acceptance because considerable excess torques arise during operation, which can result in overloading of the transmission to the point of destruction. Moreover, it was not possible to support mill operation with the required capacity in the event of the failure of a drive.